The death of 10 children in an apparent cluster bomb attack near Damascus has been widely condemned by human rights groups, which claim that the outlawed weapons have been increasingly used by the Syrian regime against civilians over the past two months.

Images of the dead and wounded children were uploaded to the internet by residents of the town of Deir al-Asafir, hours after a vacant block of land where children had gathered was hit.

The uploaded videos also showed scores of spent cluster bomblets, with the shells they were discharged from. Small indentations in the ground, where some of the bomblets had landed, were also visible, along with a large shell embedded deep in the soil.

"May God punish you, Bashar," one of the residents is heard saying of the Syrian president, Bashar al-Assad, as a video pans across a room of dead children, many of them wrapped in white shrouds. Residents contacted in the town claimed that up to 40 people, some of them children, had been wounded.

Opposition groups have insisted since late in the summer that the Syrian regime has been using the banned weapon – a claim that has been denied by officials in Damascus.

There was no independent confirmation of the attack. However, numerous images of spent shell casings have now been published from opposition-held parts of the country. None had so far had the visceral effect of the gruesome footage from Deir al-Asafir, which is believed to mark the first time in the Syrian conflict that cluster bombs have killed a large number of victims.

"It is the first confirmed video showing casualties that we have seen of what has been an increasingly clear use of these munitions," said Peter Bouckaert, emergencies director of Human Rights Watch. "The vast majority of the casualties have been caused by ordinary dumb bombs. These are very different. They are Soviet-made weapons and they are dropped from jets, which is a clear indication of who's responsible, because the opposition doesn't have warplanes."

Bouckaert said the weapons visible in the video were clearly identifiable as an AO1-SCH weapon, made by the Soviet Union in the 1970s and 80s.

"This should be seen in the context of the continued escalation by the regime and the move towards heavier weapons as they try to reclaim streets that they are slowly losing control of," Bouckaert said. "It's becoming more and more difficult for them to move around."

Cluster bombs have become an increasing menace around Aleppo and in the nearby countryside, which is a stronghold of the armed opposition. Earlier this month one such bomb fell 20 metres from a car in which the Guardian was travelling on a road near Aleppo. A jet roared overhead. Further down the road, small parachutes illuminated by flares were clearly visible dropping to the ground. Hanging beneath them were the small, drink can-sized bomblets, which detonate over a 15-20-metre radius when they hit the ground.

Russia has continued to supply Syria with weapons throughout the 20-month war, although it says they are existing supply contracts. Moscow has remained resolute in its support of its cold war ally as the crisis has escalated.

A US official quoted by ProPublica said the cash injection was highly significant. Daniel Glaser, assistant secretary of the US Treasury's office of terrorist financing and financial crimes, said: "Having currency that you can put into circulation is certainly something that is important in terms of running an economy and more so in an economy that is becoming more cash-based as things deteriorate. It is certainly something the Syrian government wants to do, to pay soldiers or pay anybody anything."

Elsewhere in Syria, rebel groups claimed to have seized control of the Tishreen dam in the north of the country. The armed opposition is steadily consolidating its presence in the Idlib and Aleppo countrysides, while Aleppo city remains locked in a stalemate that splits it roughly in half.